A rack and pinion steering gear unit is used as a mechanism for converting rotating motion that is input from a steering wheel to linear motion for applying a steering angle. Steering apparatuses that comprise a rack and pinion steering gear unit are well known, and have been disclosed for example in JP61-129670(U), JP2005-96591(A) and JP2009-56827(A). FIG. 14 illustrates construction of an example of a steering apparatus in which a rack and pinion steering gear unit is assembled. In this steering apparatus, in order to reduce the operating force for operating the steering wheel 101, auxiliary steering force from a motor 102 that is installed in the middle of the steering column 195 is applied to the steering shaft. The movement of the steering shaft that rotates as the steering wheel 101 is operated is transmitted to an intermediate shaft 106, which causes a rack shaft of a rack and pinion steering gear unit 103 to reciprocate by way of a pinion shaft 107, and this steers the steered wheels by way of tie rods 104.
As illustrated in FIG. 15, the steering gear unit 103 comprises a rack shaft 109 and a gear housing 110 that houses the rack shaft 109 so as to be able to displace in the axial direction therein. The gear housing 110 is integrally formed by performing die casting of a metal such as an aluminum alloy. By making the gear housing 110 using metal in this way, it is possible to receive the steering reaction force that is transmitted from the wheels when steering. The gear housing 110 has a cylindrical shaped main housing section 111 that is open on both ends in the lengthwise direction, and a sub housing section 112 that is provided on the outer circumferential surface of the portion near one end in the lengthwise direction of the main housing section 111, and has a center axis that is in twisted position relationship with the center axis of the main housing section 111. The internal spaces of the main housing section 111 and the sub housing section 112 are connected with each other. The gearing housing 110 is supported by and fastened to the vehicle body (frame) by bolts or studs that are inserted through a pair of installation flanges 113 that are provided at two locations around the outer circumferential surface of the main housing section 111 that are separated in the axial direction.
The rack shaft 109 comprises rack teeth that are formed on the part of the side surface in the axial direction. Except for the portion where the rack teeth are formed, the rack shaft 109 is a circular rod shape with the outer circumferential surface thereof being a cylindrical surface. With pinion teeth that are formed on the tip end section of the pinion shaft 107 engaged with the rack teeth of the rack shaft 109, the pinion shaft 107 is supported by the sub housing section 112 so as to be able to rotate. A cylinder section 108 that is equipped with a pressing block is provided on the portion in the radial direction of the main housing section 111 on the opposite side from the sub housing section 112.
In this kind of rack and pinion steering gear unit, construction is such that when the operator operates the steering wheel 101, the rack shaft 109 moves in the axial direction, and when reaching the stroke end, ball joint sockets 114 that are fastened to both ends of the rack shaft 109 in the width direction of the vehicle come in contact with the both ends of the main housing section 111 of the gear housing 110 in the width direction of the vehicle, and as a result, movement of the rack shaft 109 is stopped, and the movement distance of the rack shaft 109 is limited.
In the case of this kind of construction, the length of the gear housing 110 in the width direction of the vehicle becomes long due to the wide space in the width direction of the vehicle between the ball joint sockets 114 that are fastened on both ends of the rack shaft 109. Therefore, the thrust load when the ball joint sockets 114 come in contact with the both ends of the main housing section 111 of the gear housing 110 is supported by these both ends of the gear housing 110 that is long in the width direction of the vehicle, so it is necessary that the gear housing 110 have large strength. In order for that, the gear housing 110 must be formed using metal material having large rigidity, and the thickness of the gear housing 110 must be thick. As a result, there are problems in that the weight of the gear housing 110 increases and the manufacturing cost increases.
Furthermore, because the rack shaft 109 is long in the axial direction, there is a problem in that when integrally forming the gear housing 110 by casting, the material cost and processing cost increases.
JP61-129670(A), JP2009-56827 and JP2005-96591 disclose construction wherein the steering gear housing is divided into two or more members. Moreover, JP2005-96591(A) discloses construction wherein the gear housing is divided into three parts, left and right mounting sections that are mounted to the vehicle body, and a middle section; and of these, the left and right mounting sections are formed of an aluminum material, and the middle section is made of a steel material. With this kind of construction, the length in the axial direction of the left and right mounting sections is shorter than the length in the axial direction of the integrated gear housing, so casting work becomes easier, as well as other processing also becomes easier. However, even in the case of construction wherein the gear housing is divided, thrust load when the boll joint sockets come in contact with the gear housing is supported by both ends of the gear housing in the axial direction of the vehicle, so the gear housing still must have large strength. In order for this, it is necessary that the thickness of the gear housing be thick, and gear housing must be made of a metal material having large strength. As a result, it is not possible to solve the problem of increased weight and increased manufacturing cost of the gear housing.